Initiation of E. coli chromosome replication cannot occur in the absence of protein synthesis. After a period of thymine starvation, however, several rounds of DNA replication can be initiated despite the presence of chloramphenicol. This continued replication in the absence of protein synthesis, termed stable DNA replication, has been attributed to an altered replication apparatus formed during thymine starvation. Using an autoradiographic technique, mutants which are capable of stable DNA replication without inducing treatments such as thymine starvation have been isolated (constitutive stable replication mutants). The mutants appear to grow normally under standard growth conditions. Autoradiographic studies have demonstrated that more than 90 percent of the cells in constitutive mutant populations are capable of stable replication, but the rate of stable replication in the individual mutant cells is varied from mutants to mutants and always slower than that in the fully induced wild-type cell. One of the mutations has been mapped near the proB gene on the E. coli chromosome map. Complementation experiments using F' strains have revealed that the mutation is recessive. BIBLIOGRAPHIC REFERENCES; Kogoma, T. and Lark, K. G. (1975). J. Mol. Biol. 94, 243-256. Characterization of the replication of Escherichia coli DNA in the absence of protein synthesis: stable DNA replication. Kogoma, T. (1976). J. Mol. Biol. (in press). Two types of temperature-sensitivity in DNA replication of an Escherichia coli dnaB mutant.